I wouldn't know exactly but perhaps you can take a look at some online classifieds and compare the prices for yourself.

Here is a link that might be useful: furniture for sale

From what I have read, a sofa loses alot of its resale value as soon as it leaves the store. This is true even for premium brands. If the sofa is in excellent condition, 1/3 to 1/2 of the original price would be reasonable. If you think about it, is it reasonable to give $600 for a 3 year old piece of used furniture that you can get new for $850? If you look at places like craigslist, you will find that people think WAY too much of their used furniture.